US President Donald Trump said in a tweet Friday that Amazon's shipping rates with the US Postal Service are making the independent federal agency "dumber and poorer" while enriching the company.

"Why is the United States Post Office, which is losing many billions of dollars a year, while charging Amazon and others so little to deliver their packages, making Amazon richer and the Post Office dumber and poorer? Should be charging MUCH MORE!" he wrote in the tweet Friday morning.

The jab was the latest in a string of tweeted complaints from Trump about Amazon and its founder CEO Jeff Bezos. Trump has previously criticized Amazon for harming other retailers and not paying "internet taxes."

The USPS is an agency of the federal government -- one of the few outlined in the Constitution -- but receives no tax dollars. It relies on the sale of postage, products and services to fund its operations, and has run at a loss for years. In its latest fiscal year ended in September, the USPS reported a $2.7 billion net loss, slightly narrowing from the $2.8 billion loss a year earlier.

Amazon reported a $1.176 billion profit in the the nine months ended in September.

E-commerce giant Amazon is one of the USPS's biggest single customers, prompted the agency to offer specialized services. The USPS instituted Sunday deliveries of Amazon packages in 2013, for example, despite its longtime practice of suspending mail deliveries on the first day of the week.